Therapeutic ultrasound systems have proven effective in enhancing transdermal drug delivery, ablating pathological tissue and non-invasively breaking up concretions within the body. To achieve maximum therapeutic benefits, it is desirable to deliver ultrasound energy as directly as possible to the treatment site. Unfortunately, such treatment site may be within a body lumen, such as a vascular site, where numerous problems exist in attempting to direct therapeutic ultrasound. For example, it is difficult to design a sufficiently flexible device to deliver ultrasound energy along the curved tortuous path of the body lumen, especially for narrow diameter body lumens.
Moreover, to deliver maximum therapeutic benefits along a body lumen treatment region, it is desirable to direct a uniform dosage of ultrasonic energy along the length of the lumen with the dosage of the ultrasound energy varying only minimally along the length of the lumen. Delivering a uniform dose of therapeutic ultrasound energy along the length of the body lumen is especially desirable when concurrently using stents in the lumen. When using stents, overstretching of the vascular wall during stent insertion can cause wall tearing and denudation of endothelial cells which can result in an over proliferative healing response. Therapeutic ultrasound following wall injury reduces the formation of obstructive neointimal hyperplasia. A uniform dose of therapeutic ultrasound would reduce the formation of such hyperplasia along the length of the lumen, and in particular along the length of the stent.
It has proven especially difficult to generate such a uniform ultrasonic field along the length of a body lumen due in part to the typically curved path of the lumen and the dimensions of the ultrasound transducers.
Ultrasound systems which are effective in enhancing transdermal drug delivery operate at frequencies around 1MHz, and tend to be quite large due to the large surface area that it is necessary to affect. Such large transducers are not suitably dimensioned for catheter placement into the small lumens of a patient's body. Moreover, smaller transducers which operate at higher frequencies, (such as 10 to 50 MHZ), are not adapted to generate sufficient energy to enhance in vivo drug delivery, or to cause other therapeutic effect, such as reducing the formation of obstructive neointimal hyperplasia after stent implantation. Instead, such small high frequency transducers are limited to diagnostic applications.
For catheter based systems, achieving the optimal size of the ultrasound transducer is problematic since a small catheter mounted transducer is only able to deliver a small amount of ultrasound energy to the patient. Conversely, a larger device, (which would deliver more therapeutic energy), requires a larger transducer which would unfortunately limit the flexibility of the catheter, thus making access difficult in narrow vascular regions.
In addition, a small catheter mounted transducer is adapted to deliver ultrasound only to the region of the lumen immediately adjacent the transducer, for example at the distal tip of the catheter. An additional problem when using a plurality of ultrasound transducers spaced apart along the length of the catheter is the non-uniformity of ultrasound dose delivered since maximum ultrasound will be delivered adjacent the transducers and minimal ultrasound will be delivered at locations equally spaced between adjacent transducers. Accordingly, it is especially difficult to deliver a uniform dose of ultrasound energy along the length of the body lumen.
U.S. Pat. No. 5,197,946 and published PCT Applications WO 96/27341 and WO 98/18391 to Tachibana disclose catheters having an ultrasound transducer at their distal end. Published PCT Application WO 98/48711 to Tachibana discloses a flexible catheter system directed to providing ultrasound for treating long lesions by providing a catheter having a number of separate ultrasound transducers spaced apart therealong. Published PCT Application WO 96/29935 to Crowley discloses a catheter system for tissue ablation having a plurality of annular shaped ultrasonic transducers spaced apart along the length of the catheter.